The invention is based on an electronic ballast according to the preamble of claim 1.
An electronic ballast principally contains an AC voltage generator, which provides an AC voltage of an oscillation frequency that is substantially higher than the frequency of the mains voltage. Through suitable means, the electronic ballast must start an attached discharge lamp and subsequently make it operate. The starting of a discharge lamp with electrode filaments can be subdivided into preheating and ignition. For preheating, a current flows through the electrode filaments and brings them to a temperature which allows subsequent ignition that entails only minor damage to the electrode filament. Once the discharge lamp has been ignited, the operation of the discharge lamp begins. In this state, the current for the gas discharge in the lamp is supplied via the electrode filament terminals. A current which flows in at one electrode filament terminal is then divided into a part that flows into the gas discharge and a part that flows out again at the other terminal of the same electrode filament. The part of the current which does not flow into the gas discharge causes additional heating of the electrode filament in relation to the gas discharge, for which reason this current is referred to as an auxiliary heating current. In discharge lamps with fragile electrode filaments, this auxiliary heating current must be low in order to achieve a long life. It is therefore expedient for the electrode filaments to carry essentially the gas discharge current during operation. The auxiliary heating current should be small compared with the current for the gas discharge (at most 20%).
The document EP0748146 (Krummel) proposes a heating transformer for the preheating. The AC voltage generator feeds the preheating current into its primary winding. Each electrode filament is attached to a secondary winding of the heating transformer. A switch can be used to interrupt the flow of current in the primary winding of the heating transformer. This makes it possible to prevent any auxiliary heating current from flowing during operation, so that the electrode filaments carry essentially the gas discharge current. However, this solution requires a switch and the system needed for actuating it.
It is an object of the present invention to provide an electronic ballast according to the preamble of claim 1, which does not need a switch for turning off the preheating and is therefore less expensive than the aforementioned solution.
This object is achieved, in the case of an electronic ballast having the features of the preamble of claim 1, by the features of the characterizing part of claim 1. Particularly advantageous configurations are given in the dependent claims.
According to the invention, the electronic ballast contains a frequency-selective device, which permits preheating of the electrode filaments only if the oscillation frequency of the electronic ballast is within a narrow frequency band, which is determined by the frequency-selective device. The frequency-selective device is preferably formed by a tuned circuit consisting of an inductor L and a capacitor C. The electronic ballast""s oscillation frequency, at which preheating is then possible, is given by the resonant frequency of this tuned circuit, where:       f    res    =      1          2      ⁢      π      ⁢              LC            
Of course, it is not necessary to comply exactly with this frequency. Instead, it is sufficient for the electronic ballast""s preheating oscillation frequency to be within a narrow frequency band around the resonant frequency fres. In practice, it has been found that a frequency band of +/xe2x88x9210% around the resonant frequency is sufficient.
The inductor of the tuned circuit may, according to the invention, be formed by the primary inductor of the heating transformer. That is to say, the inductor which constitutes the primary winding of the heating transformer, together with a series resonance capacitor connected in series or a parallel resonance capacitor connected in parallel, forms the tuned circuit. In principle, the tuned circuit may also be formed on the secondary side of the heating transformer. Since the impedance level is lower there, however, high resonant currents that constitute a high component load are produced.
In order to be able to form a sufficiently high primary inductance, a heating transformer with loose coupling may be selected.
Since the AC voltage generator of the electronic ballast often delivers a square-wave voltage, the tuned circuit can also be excited by harmonics. Since, according to the invention, no auxiliary current is meant to flow during operation of the lamp, the electronic ballast must not, at the operating frequency, excite the tuned circuit with a harmonic. The term xe2x80x9coperating frequencyxe2x80x9d is intended to mean the oscillation frequency at which the electronic ballast functions during operation of the lamp. Since a square-wave oscillation has only odd harmonics, the tuned circuit is configured, according to the invention, in such a way that its resonant frequency will be at, or close to, two times the operating frequency. The inventive concept is still achieved if the resonant frequency fres of the tuned circuit is two times the operating frequency to within a tolerance of +/xe2x88x9220%.